The long term objective of this proposed study is to determine the organization and anatomical relationships of enkephalin and serotonin circuits in a specific cardiovascular nucleus, the nucleus tractus solitarii, as they possibly relate to regulation of cardiovascular functions. The specific aims of this proposal are 1) to determine the precise termination of enkephalin immunoreactive neurons which are contained within cardiovascular related subdivisions of the nucleus tractus solitarii of the cat, 2) to investigate the synaptic relationships and morphological aspects of enkephalin immunoreactive terminals and neurons as they relate to visceral afferents terminating in the nucleus tractus solitrarii and 3) to describe the synaptic relationships and patterns of serotonergic terminals and enkephalin containing neurons and terminals. In the case of possible projections of enkephalin containing neurons and serotonergic input to these neurons, the concurrent use of HRP retrograde tract-tracing will be used in combination with neurotransmitter immunohistochemistry both at the light and electron microscopic levels. Results of the proposed study are expected to provide the morphological basis for the enkephalinergic and serotonergic effects observed within the nucleus tractus solitarii and its projections in relation to central regulation of cardiovascular function. Furthermore, these studies will contribute important information concerning the neurons and circuits that are involved in central control of autonomic functions.